Mobile devices are being required to support multiple applications, such as GSM, PSC, UMTS, WLAN, Wibro (Wireless broadband), and Bluetooth, and LTE, which in turn require multiple antennas, since one antenna cannot typically cover the bandwidth requirements of the multiple applications due to the physical limitations of an antenna described in “Physical Limitations of Antennas,” IEEE Transactions on Antennas and Propagation, vol. 51, no. 8, pgs. 2116-2123, 2003. As a result, multiple antennas must now share the already limited space within the mobile device.
Furthermore, techniques such as multiple-input multiple-out (MIMO) have emerged, which significantly increase the performance of HSPA (high speed packet access) and LTE (long term evaluation) networks. This is usually accomplished by using multiple antennas arranged to have low correlation between two or more unique radio signals. In large devices, where space is less limited, this is easily accomplished by using spatial diversity (distance between antennas), or somehow by pattern diversity (difference between antenna aiming directions), and polarization diversity together. Unfortunately, the size of mobile wireless communications devices (e.g., cellular devices) continue to decrease and so too does the allowable space for the device antenna. As a result, having multiple antennas in a close proximity poses significant coupling and mode isolation problems; furthermore, the signals received by each of the antennas may be undesirably correlated. This noticeably disrupts MIMO performance.
Thus it can be seen that designers of antennas for mobile devices face significant challenges, particularly wherein the antennas may be capable of covering as many bands as possible while being small in size and still having a high performance.
One form of antenna commonly used in mobile devices is the monopole antenna. Compared to PIFA or IFA, a monopole can easier achieve large bandwidth because they may be arranged to radiate at two or more resonant frequencies (from its fundamental mode, second order and higher modes) Since a monopoles inherent dual mode characteristic makes it easy to achieve a frequency ratio of two-to-one of its upper and lower frequency band.
However using a single radiator for multi-order modes poses a difficulty, particularly if specific frequency bands are to be adjusted independently. Additionally, in a single radiator if one of the operating bands is required to be relatively wide the monopole may not cover all bands, such as GSM 900 (880 to 960 MHz) at a lower band and GSM1880/1900 and UMTS2100 (1710 to 2170 MHz) together at an upper band, unless additional parasitic branches are used to enhance the bandwidth and adjust the frequency ratio. However this introduces additional volume and potential higher-mode coupling among radiation elements.
Another disadvantage is that since a monopole is typically a quarter-wavelength of the fundamental mode, the size of the antenna is increased when it is designed to operate at lower resonant frequency bands.
Accordingly, it is desirable to have a monopole that may be arranged in a limited space.